Try this code...
<?php
$currencyBlock = Mage::app()->getLayout()->createBlock('directory/currency');
if($currencyBlock->getCurrencyCount() > 1): ?>
<div class="form-language">
<label for="custom-currency-selector"><?php echo $this->__('Your Currency:') ?></label>
<select onchange="window.location.href=this.value" name="custom-currency-selector" id="custom-currency-selector">
<?php foreach ($currencyBlock->getCurrencies() as $_code => $_name): ?>
<option value="<?php echo $currencyBlock->getSwitchCurrencyUrl($_code)?>"
<?php if($_code == $currencyBlock->getCurrentCurrencyCode()): ?>
selected="SELECTED"
<?php endif; ?>>
<?php echo $_code ?>
</option>
<?php endforeach; ?>
</select>
</div>
<?php endif; ?>
if you don't want to use currency code in header.phtml and want to manage in separate phtml file then you can use below code in header.phtml:
<?php echo $this->getLayout()->createBlock('directory/currency')->setTemplate('directory/currency.phtml')->toHtml();?>
you can find phtml file here: app/design/frontend/[Current_Package]/[Current_Theme]/template/directory/currency.phtml
if you still not able to find you can see currency.phtml in here : app/design/frontend/base/default/template/directory/currency.phtml
Hope this helps you. All the best!